1. Field of the Invention
The present invention relates to an image forming apparatus and more specifically to a method for calibrating a toner image detection sensor that reads the density of a reference toner image formed on a photosensitive drum or an intermediate transfer belt.
2. Related Art
In recent years, electrophotographic image forming apparatuses such as color copiers and color printers that enable multicolor image formation have been developed and, for example, color image forming apparatuses using an intermediate transfer system are well known, in which image formation is performed by forming a toner image of each color on a latent image carrier such as a photosensitive drum, then forming a multicolor image through sequential superimposition and transfer of those toner images of respective colors onto an intermediate transfer belt, which is an intermediate transferer, and then transferring and fixing the multicolor image on recording paper, which is transfer paper.
In such image forming apparatuses, toners are primarily transferred onto an intermediate transfer belt, the densities of the transferred toners are read by an optical sensor (toner image detection sensor) that includes a light emitting device and a light receiving device, and a developing bias is changed according to the toner densities that have been read so as to perform correction such as high density correction and gray-level correction. Registration correction (color shift correction) is also performed in a similar way.
The optical sensor that reads toners transferred on the intermediate transfer belt usually performs its own calibration in order to increase the precision of reading. As a method for performing the calibration, there is a conventional method in which a default value (Yd) for a current value applied to the light emitting device of the optical sensor is stored in advance and used for calibration, as indicated by the dashed-dotted line in FIG. 10. FIG. 10 is an explanatory drawing showing the time required for calibration of the optical sensor in cases where the calibration is performed in a manner according to the present invention (indicated by the solid line) and where the calibration is performed in the conventional manner (indicated by the dashed-dotted line).
Another image forming apparatus has also been suggested, which is configured to, instead of using a default value as described above, correct a reference current value and a reference voltage value by reference to the output of a temperature and humidity sensor and an environmental compensation table so that optimum current and voltage values are output to a transfer roller (see JP 2005-134417A, which is hereinafter referred to as “Patent Document 1”).
Ordinarily, optical sensors are highly temperature dependent. However, in the above-described conventional method for performing calibration using a default value (Yd), since no consideration is given to the temperature characteristics of the optical sensor, calibration needs to be retried many times, depending on the ambient temperature (environmental temperature) around the optical sensor at the time of execution of the calibration, and so adjustment of the calibration takes time.
In other words, referring to the conventional calibration example indicated by the dashed-dotted line in FIG. 10, in a case where a sensor output voltage (X11) of the light receiving device acquired by applying a default value (Yd) of current to the light emitting device of the optical sensor deviates from an appropriate value range Xw (e.g., a range of 2.5 to 2.6 V) of the sensor output voltage, calibration for changing the current value applied to the light emitting device by a predetermined value is performed four times in order to make the sensor output voltage within the appropriate value range Xw.
Also, the method described in Patent Document 1 is a method for selecting a correction value from the environmental compensation table, using the correction value to correct the reference current value and the reference voltage value, and outputting the corrected reference current value and the corrected reference voltage value as final values to the transfer roller and a suction roller. That is, while the environmental compensation table is used to correct the reference current value and the reference voltage value, it is not used for the calibration of the optical sensor itself, so that the problem still remains that adjustment of the calibration of the optical sensor takes time as in the case of the conventional technique.